The two-shot COVID vaccines require two shots for a good reason: study

Some of the vaccines use new technology, so it is unclear if they'll be as effective if not fully administered

By Matthew Rozsa
Published July 30, 2021 5:30AM (EDT)
A doctor prepares syringes that contain the Pfizer/BioNTech vaccine against Covid-19 at a mobile vaccination center in the Markkleeberg suburb town hall on May 10, 2021 in Leipzig, Germany. Germany has succeeded in accelerating its nationwide vaccinations in recent weeks. Approximately one third of the population has received a first dose. (Jens Schlueter/Getty Images)
A doctor prepares syringes that contain the Pfizer/BioNTech vaccine against Covid-19 at a mobile vaccination center in the Markkleeberg suburb town hall on May 10, 2021 in Leipzig, Germany. Germany has succeeded in accelerating its nationwide vaccinations in recent weeks. Approximately one third of the population has received a first dose. (Jens Schlueter/Getty Images)

In April, the Centers for Disease Control and Prevention found that millions of Americans who received one shot of the two-dose Pfizer and Moderna vaccines never returned for their second jab. Patients gave explanations ranging from patients mistakenly believing they only needed one dose and being wary after their initial inoculation led to side effects to having suppliers who ran out of what they needed. Either way, a new study makes it clear that getting only one shot of a two-dose vaccine is a pretty bad idea.

"This is the first time RNA vaccines have ever been given to humans," said Dr. Bali Pulendran, who described the two mRNA vaccines' "95% protection against COVID-19" as remarkable. Pulendran is a professor of pathology and of microbiology and immunology at Stanford University, who co-authored a study in the journal Nature about the effectiveness of Pfizer's two-shot vaccine after only one jab. 

"Despite their outstanding efficacy, little is known about how exactly RNA vaccines work," he noted. And although the new vaccines have so far seemed to work remarkably well — and are incredibly safe — the data does not suggest that one shot of two will protect one from COVID-19. 


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The scientists behind the study found, among other things, that while the first shot does boost SARS-CoV-2-specific antibody levels, getting a second dose boosts antibody levels to a far greater degree. Indeed, compared to the first shot, it is striking that the second one "stimulated a manifold increase in antibody levels, a terrific T-cell response that was absent after the first shot alone, and a strikingly enhanced innate immune response," Pulendran explained.

They also found that patients who received the Pfizer vaccine had an increase amount of a type of monocyte (a type of cell that includes a lot of antiviral genes) in their blood. The second dose in particular would stimulate a massive mobilization of this newly-discovered group of cells, which increase their presence in our blood from being only 0.01% of all circulating blood cells to constituting a full 1% of all blood cells after the second Pfizer shot.

"The extraordinary increase in the frequency of these cells, just a day following booster immunization, is surprising," Pulendran explained. "It's possible that these cells may be able to mount a holding action against not only SARS-CoV-2 but against other viruses as well."

Both Pfizer and Moderna use a novel vaccine technology that makes it much more difficult to anticipate their long-term performance than other vaccines. While other vaccines use all or part of a pathogen (disease-causing microorganism) to train the immune system how to recognize and defeat specific threats, Pfizer and Moderna insert a synthesized RNA molecule that trains cells how to produce proteins identical to those on the specific pathogen — in other words, using the body's own machinery to create a vaccine within, without ever being exposed to a dead or weakened version of said pathogen.


Matthew Rozsa

Matthew Rozsa is a staff writer for Salon. He holds an MA in History from Rutgers University-Newark and is ABD in his PhD program in History at Lehigh University. His work has appeared in Mic, Quartz and MSNBC.

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